OFDM (Orthogonal Frequency Division Multiplexing) and OFDMA (Orthogonal Frequency Division Multiple Access)-based mobile radio standards are multicarrier systems. The combination of the individual carriers results in a signal whose maximum instantaneous power considerably exceeds the mean power. Furthermore, higher-quality modulation methods, such as QPSK (Quadrature Phase Shift Keying), 8PSK (Phase Shift Keying) and 16 QAM (Quadrature Amplitude Modulation) can also be used with more than two possible information states per transmission symbol, in order to modulate the various carriers that are used. In the case of these modulation methods, constancy of the instantaneous transmission power in the individual carrier is dispensed with in favor of higher data transmission rates.
Depending on the modulation method and combination of the individual carriers, the maximum instantaneous power of a transmission signal in multicarrier systems may be considerably higher than the mean value of the transmission power. The ratio of the maximum to the mean transmission power is referred to as the peak to average ratio (PAR>1) or peak to average power ratio (PAPR>1). One problem is now that, when the PAR is relatively high, the result of limiting or clipping effects. This increases the complexity and power consumption of the amplifier, and reduces the efficiency of the overall system.
By way of example, DE 10219318 A1 relates to a method for forming a carrier-frequency output signal from a plurality of partial signals of a base frequency band. Each partial signal is amplitude-limited in a main branch. Furthermore, estimated values are determined for each partial signal in a parallel secondary branch, by which estimated values the amplitude limiting in the main branch is controlled.
DE 19838295 A1 describes a multicarrier transmission method for parallel transmission of information in the form of a signal on a plurality of subchannels. A correction function is in each case applied by a threshold value decision to subchannels which are precluded from information transmission, such that a ratio between a peak value of the signal and the root mean square value of the signal is minimized or at least reduced.
EP 1248429 A2 describes a DMT (Discrete Multitone) modulation method which allows the ratio of the maximum to the mean transmission power of a signal to be transmitted to be reduced. For this purpose, a trial value of a limiting prevention signal is established first of all, and a determination is made as to whether the limiting prevention signal is suitable for limiting the signal to be transmitted. If this is not the case, then the limiting prevention signal is varied until it is suitable for limiting the signal to be transmitted. The limiting prevention signal, which in the end is suitable, is transmitted using unused subcarriers of the signal to be transmitted, in order to reduce the peak power values of the signal to be transmitted. An iteration loop is used in order to determine the suitable limiting prevention signal. However, this can lead to different signal delays since it is not known from the start how often the iteration loop will be passed through.